The present invention relates to method and apparatus specimen fabrication for analyzing, observing, or measuring a micro area by separating a micro sample including a requested specific area from a sample of an electronic part such as a semiconductor wafer or a device or preparing the micro sample to be separated by using a focused ion beam.
Electronic parts such as a semiconductor memory typified by a dynamic random access memory, a microprocessor, a semiconductor device such as a semiconductor laser, and a magnetic head are required to be manufactured in a high yield since decrease in the manufacturing yield due to occurrence of a defect causes profit deterioration. Consequently, early detection/measure of/against a defect, a foreign matter, and poor processing as causes of a failure are big tasks. For example, at a site of manufacturing a semiconductor device, energies are put into finding a failure by a careful test and analyzing the cause of the failure. In an actual electronic part manufacturing process using a wafer, a wafer being processed is tested, the cause of an abnormal portion such as a defect in a circuit pattern or a foreign matter is tracked down, and a measurement to be taken is examined.
Usually, to observe a fine structure of a sample, a scanning electron microscope (hereinbelow, abbreviated as SEM) with high resolution is used. However, as the packing density of a semiconductor device is becoming higher, an object cannot be observed with the resolution of the SEM. Therefore, in place of the SEM, a transmission electron microscope (hereinbelow, abbreviated as TEM) having higher observation resolution is used.
Conventional TEM sample fabrication is accompanied by a work of making a sample into small pieces by cleaving, cutting, or the like. When the sample is a wafer, in most cases, the wafer has to be cut.
Recently, there is a micro area processing method of irradiating a sample with an ion beam and applying an action that particles constructing the sample are released from the sample by sputtering, that is, a method of using a process with a focused ion beam (hereinbelow, abbreviated as FIB). According to the method, first, a strip pellet having a thickness of sub millimeters including an area to be observed is cut from a sample such as a wafer by using a dicer or the like. A part of the strip pellet is processed with an FIB into a thin film state to thereby prepare a TEM sample. The feature of the sample for TEM observation processed with the FIB is that a part of a specimen is processed to a thin film having a thickness of about 100 nm for the TEM observation. Although the method enables a requested observation area to be positioned with accuracy of a micrometer level and to be observed, still, the wafer has to be cut.
Although monitoring a result of a process during fabrication of a semiconductor device or the like has an big advantage from the viewpoint of managing the yield, a wafer is cut for preparation of a sample as described above and a piece of the wafer is not subjected to a following process but is discarded. In recent years, particularly, the diameter of a wafer is increasing to reduce the price of manufacturing a semiconductor device. Specifically, the number of semiconductor devices which can be manufactured from a single wafer is increased to reduce a unit price. However, it increases the price of the wafer itself, an added value increases as the manufacturing process advances and, further, the number of semiconductor devices lost by discarding a wafer increases. Therefore, the conventional test method accompanying cutting of the wafer is very uneconomical.
To deal with the problem, there is a method of preparing a sample without cutting a wafer. The method is disclosed in Japanese Patent Application No. H05-52721, “Method of separating sample and method of analyzing sample separated by the separating method” (known technique 1). According to the method, as shown in FIGS. 2(a) to 2(g), first, the posture of a sample 2 is maintained so that the surface of the sample 2 is irradiated with an FIB 1 at the right angle and scanned with the FIB 1 in a rectangular shape, and a rectangular hole 7 having a required depth is formed in the surface of the sample (FIG. 2(a)). Subsequently, the sample 2 is tilted and a bottom hole 8 is formed. The tilt angle of the sample 2 is changed by a specimen stage (not shown) (FIG. 2(b)). The posture of the sample 2 is changed, the sample 2 is disposed so that the surface of the sample 2 becomes perpendicular to the FIB 1 again, and a trench 9 is formed (FIG. 2(c)). By driving a manipulator (not shown), the tip of a probe 3 at the end of the manipulator is made come into contact with a portion to be separated in the sample 2 (FIG. 2(d)). A deposition gas 5 is supplied from a gas nozzle 10, and an area including the tip of the probe 3 is locally irradiated with the FIB 1 to form an ion beam assist deposition film (hereinbelow, simply called deposition film 4). The separation portion in the sample 2 and the tip of the probe 3 which are in contact with each other are connected to each other by the deposition film 4 (FIG. 2(e)). The peripheral portion is trenched with the FIB 1 (FIG. 2(f)), and a micro sample 6 as a sample separated from the sample 2 is cut. The cut separated sample 6 is supported by the connected probe 3 (FIG. 2(g)). The micro sample 6 is processed with the FIB 1 and the area to be observed is walled, thereby obtaining a TEM sample (not shown). According to the method, a micro sample including a requested analysis area is separated from a sample such as a wafer by using a process with an FIB and means for carrying the micro sample. The micro sample separated by the method is introduced to any of various analyzers and can be analyzed.
A similar sample fabricating method is disclosed in Japanese Patent Application Laid-Open No. H09-196213, “Apparatus and method for preparing micro sample” (known technique 2). According to the method, as shown in FIGS. 9(a) to 9(j), first, the FIB 1 is emitted to form marks 403 and 404 for identifying a target position and, after that, rectangular holes 401 and 402 are formed on both outer sides of the marks 403 and 404 in the sample 2 (FIG. 9(a)). Subsequently, a trench 406 is formed with the FIB 1 (FIG. 9(b)). The specimen stage is tilted and the surface of the sample is obliquely irradiated with the FIB 1, thereby forming a tapered trench 408, and an extraction sample 407 which is connected to the sample 4 only via a residual area 405 is formed (FIG. 9(c)). The tilted specimen stage is returned to the original position and the probe 3 is controlled by a probe controller so as to come into contact with a part of the extraction sample 407. The residual area 405 of the extraction sample 407 will be cut with an FIB later. In consideration of a probe drift or the like, it is desirable to cut the residual area 405 in short time, so that the volume of the residual area 405 has to be low. Consequently, due to a fear that the residual area 405 is destroyed by the contact of the probe 3, the probe 3 is made contact while preventing a damage as much as possible by using the probe controlling method. The probe 3 and the extraction sample 407 which are in contact with each other are fixed by using a deposition film 409 (FIG. 9(d)). Subsequently, the residual area 405 is cut with the FIB 1 (FIG. 9(e)). In such a manner, the extraction sample 407 is cut out, and the probe 3 is lifted by the probe driving apparatus to extract the extraction sample 407 (FIG. 9(f)). Subsequently, the cut extraction sample 407 is allowed to come into contact with a trench 411 formed in an extracted sample holder (FIG. 9(g)). At this time, the extraction sample 407 has to come into contact at a sufficiently low speed so that the extraction sample 407 is not destroyed or is not come off from the connected portion with the deposition film 409, so that the contacting method is necessary. After making the extraction sample 407 contact with the trench 411, they are fixed by using a deposition film 412 (FIG. 9(h)). After the fixing, the probe 3 connection portion is irradiated with the FIB, and sputtering is performed to separate the probe from the extraction sample 407 (FIG. 9(i)). In the case of preparing a TEM sample, finally, the FIB 1 is emitted again to finish an observation area 410 so that the thickness of the observation area 410 becomes about 100 nm or less (FIG. 9(j)). In the case of preparing a sample for analysis or measurement, the finishing process for making the observation area thin (FIG. 9(j)) is not always necessary.
The example of employing the method of extracting a micro sample by the sample fabricating apparatus has been described above. There is also a method of processing the shape of a micro sample by the sample fabricating apparatus, taking out the base from the sample fabricating apparatus, and extracting the micro sample by another mechanism in atmosphere. For example, such a method is described by L. A. Giannuzzi et al., “Focused Ion Beam Milling and Micromanipulation Lift-Out for Site Specific Cross-Section TEM Specimen Preparation”, Material Research Society, Symposium Proceeding Vol. 480, pp. 19 to 27 (known technique 3). Similarly, it is also descried by L. R. Herlinger, “TEM Sample Preparation Using a Focused Ion Beam and a Probe Manipulator”, Proceedings of the 22nd International Symposium for Testing and Failure Analysis, pp.199 to 205 (known technique 4).
According to such a method, as shown in FIG. 3(a), both sides of a target position on a wafer 208 are processed in a stair shape with the FIB 1 to form a sample membrane 207, a specimen stage is tilted to change the angle formed between the FIB 1 and the surface of the sample, and the sample is irradiated with the FIB 1. As shown in FIG. 3(b), the periphery of the sample membrane 207 is cut with the FIB 1, thereby separating the sample membrane 207 from the wafer. The wafer is taken out from an FIB system, a glass stick is allowed to approach the process portion in the atmosphere, the sample membrane 207 is attracted by the glass stick by using static electricity and is separated from the wafer, the glass stick is moved above a mesh 209 and is attracted by the mesh 209 by using static electricity or disposed so that the process face faces a transparent attachment. As described above, the processed micro sample in the system may not be taken out in the system. Even when most of the outer shape of the micro sample is processed with an ion beam, the separated micro sample is introduced into the TEM, and can be analyzed.
By using any of the methods, without cutting a wafer, only a micro sample or a membrane sample for test is extracted from a sample, and the wafer from which the sample is extracted can be returned to the next process. Therefore, unlike the conventional techniques, there is no semiconductor device which is lost by the cutting of a wafer, the manufacturing yield of the semiconductor device is increased in total, and the manufacturing cost can be reduced.
In the case of forming a hole by using sputtering of irradiating the surface of a sample with an ion beam and observing a section of the hole by an FIB system or a scanning electron microscope (SEM), the section is formed at an end of an ion beam scan range.
However, the actually formed section is not perfectly perpendicular to the surface of a sample due to flare of a processing beam and re-deposition of a sputtered substance, and a slight taper exists. An FIB system having a mechanism of tilting a specimen stage can prevent the taper by tilting a sample by an angle corresponding to the taper, for example, about 0.5 degree and irradiating the tilted sample with an ion beam and form an observation section having higher perpendicularity. The method is described as, for example, processing of a sample section of a transmission electron microscope (TEM), in “Electron and ion beam handbook, Third Edition”, Japan Society for the Promotion of Science, 132 commission, Nikkan Kogyo Shinbun Sha, pp. 459 and 460 (known technique 5).
The conventional methods have the following problems. Specifically, to form the bottom hole 8 in the first known technique, to form the tapered trench 408 in the second known technique, and to cut the periphery of the sample membrane 207 in the fourth known technique, the posture or tilt angle of the sample 2 is changed as a necessary process by the specimen stage. However, as the diameter of a wafer increases, the specimen stage also becomes larger. Consequently, a problem such that it takes time to tile a large stage with high accuracy and, as a result, sample fabrication time becomes longer arises. Due to heavy weight of the specimen stage itself, the eucentric is not maintained before and after the tilting and the sample position relative to the ion beam irradiating optical system moves, so that the focal point of the FIB is relatively largely deviated from the surface of the sample, the surface of the sample cannot be observed, and a problem such that the ion beam irradiating optical system has to be re-adjusted also occurs. The function of tilting the specimen stage causes increase in the size of the specimen stage itself and in the size of a specimen chamber for housing the specimen stage. The trend of the diameter of a wafer is shifting from 200 mm to 300 mm. When the diameter of a wafer is further increased to 400 mm, the size of the stage has to be increased and the problem which occurs in association with the tilt of the specimen stage has to be solved. In contrast, when the function of tilting the specimen stage of the system can be eliminated, miniaturization of the whole system can be realized and a problem such as a deviation of the sample position accompanying a tilt of the sample is solved. However, by the above-described conventional methods, fabrication of a sample for analyzing, observing or measuring a micro area by separating a micro sample from an original sample (wafer) or preparing the micro sample to be separated cannot be realized. Originally, the change in the tilt angle or posture of a sample is required due to existence of the fixed idea that the surface of a sample has to be irradiated with ion beams in at least two directions at different angles to separate a micro sample from an original sample or prepare the micro sample to be separated. The tilting of the stage denotes here turning of a stage around a line segment included in or parallel to the stage plane as an axis. It will be simply described as tilting of a stage hereinlater.
By an FIB controller in which a specimen stage has the tilting function, an FIB can be emitted at an arbitrary angle, and can eliminate the taper as in the known technique 5.
On the other hand, the function of tilting a specimen stage can be omitted from the system, the miniaturization of the whole system is realized, and the program such as a deviation of the sample position which occurs in association with the tilting of a sample can be solved. However, according to the conventional methods, it is difficult to emit an FIB at an arbitrary angle. A method of obliquely irradiating the surface of a sample with an ion beam to form a hole, thereby enabling an observation section to be formed is disclosed as “Section observing method” in Japanese Patent Application Laid-Open No. H03-166744 (known technique 6). Although a process of forming a vertical section by the method is described, a method of optionally changing an irradiation angle without tilting a specimen stage is not mentioned. Consequently, it is difficult to eliminate the taper.